This species has been considered a subspecies of Z. californianus, (Z. c. japonicus), by many authors. Rice (1998), followed here, argued for the retention of Z. californianus, Z. japonicus, and Z. wollebaeki as distinct species without providing new evidence. From new studies of skull morphology Brunner (2003) argued for species status. Additionally, genetic evidence that Z. japonicus is best considered a separate species is given by Sakahira and Niimi (2007) and Wolf et al. (2007).

Justification:
There have been no documented reports of Z. japonicus since the late 1950s, despite extensive marine mammal research effort taking place within its former range. The last credible report was 50 to 60 individuals on Takeshima in 1951 (Rice 1998). Individual sightings reported as recently as 1974 and 1975, cannot be confirmed; confusion with escaped Z. californianus cannot be ruled out.

This species was formerly known from the northwest Pacific where it probably occurred along the coasts of Japan, the Democratic People’s Republic of Korea, the Republic of Korea, and Russia at Sakhalin Island.

Estimates are that 30,000 to 50,000 animals may have been present in the mid-19th century. The last available abundance estimate suggested the presence of 100 animals on Takeshima Island, and a total population of up to 300 in the late 1950s. There have been no documented reports of the species since the late 1950s, and most authors now consider this species to be extinct.

Very little information exists on the appearance of this animal. In an account from otter and seal hunters working in this area in the early 20th century, the “black sea lion” was said to have been present in addition to animals that were likely to be Steller Sea Lions. The common name “black sea lion” may usefully point-out that some animals, presumably adult males, were very dark brown or black, as is the case for many adult male California sea lions. A colour plate showed, and an accompanying account from the text of a mid-19th century work gave, a description of the animal as “straw coloured with a darker throat and chest in the female.”

A Japanese zoologist interviewed in the 1950s gave the lengths of adult males as 2.5 m and adult females as 1.4 m, and reported a four-month-old pup as being 65 cm long and 9 kg. A review in the late 1950s listed eight specimens as existing in museums, with none of these in Japan.

Very little information is available on these animals, although they are assumed to be similar to the California Sea Lion. Anecdotal information suggests that the species was known to occupy coastal areas, was rarely found more than 16 km out to sea, frequently hauled out throughout the year, bred mainly on flat, open, sandy beaches, and sometimes in rocky areas, and they were said to be good divers that fed on fish. However, no detailed information or results of studies are available to support these observations.

The species probably became extinct shortly after the last reports of sightings in the late 1950s, although the very remote possibility of a remnant colony in Korean waters still exists. Japanese Sea Lions were taken for their skin and oil. Certain internal organs were also valuable in traditional medicinal practices and whiskers were reportedly used as pipe cleaners. However, the main reason for the extinction of the Japanese Sea Lion is thought to have been persecution by fishermen.

This species is almost certainly extinct. However, a comprehensive survey throughout the range of the species has not been undertaken to determine the status of the species. Additional work to uncover all available information on specimens, distribution, observations and data, and photographs from countries where the species occurred should also be started while people who observed and interacted with this species are still alive.